Offshore buoyant structures for oil and gas production, storage and offloading are known in the art. Offshore production structures, which may be vessels, platforms, caissons, buoys, or spars, for example, each typically, include a buoyant hull that supports a superstructure. The hull includes internal compartmentalization for storing hydrocarbon products, and the superstructure provides drilling and production equipment, crew living quarters, and the like.
A floating structure is subject to environmental forces of wind, waves, ice, tides, and current. These environmental forces result in accelerations, displacements and oscillatory motions of the structure. The response of a floating structure to such environmental forces is affected not only by its hull design and superstructure, but also by its mooring system and any appendages. Accordingly, a floating structure has several design requirements: Adequate reserve buoyancy to safely support the weight of the superstructure and payload, stability under all conditions, and good seakeeping characteristics. With respect to the good seakeeping requirement, the ability to reduce vertical heave is very desirable. Heave motions can create alternating tension in mooring systems and compression forces in the production risers, which can cause fatigue and failure. Large heave motions increase riser stroke and require more complex and costly riser tensioning and heave compensating systems.
The seakeeping characteristics of a buoyant structure are influenced by a number of factors, including the waterplane area, the hull profile, and the natural period of motion of the floating structure. It is very desirable that the natural period of the floating structure be either significantly greater than or significantly less than the wave periods of the sea in which the structure is located, so as to substantially decouple the motion of the structure from the wave motion.
Vessel design involves balancing competing factors to arrive at an optimal solution for a given set of factors. Cost, constructability, survivability, utility, and installation concerns are among many considerations in vessel design. Design parameters of the floating structure include the draft, the waterplane area, the draft rate-of-change, the location of the center of gravity (“CG”), the location of the center of buoyancy (“CB”), the metacentric height (“GM”), the sail area, and the total mass. The total mass includes added mass—i.e., the mass of the water around the hull of the floating structure that is forced to move as the floating structure moves. Appendages connected to the structure hull for increasing added mass are a cost effective way to fine tune structure response and performance characteristics when subjected to the environmental forces.
Several general naval architecture rules apply to the design of an offshore vessel: The waterplane area is directly proportional to induced heave force. A structure that is symmetric about a vertical axis is generally less subject to yaw forces. As the size of the vertical hull profile in the wave zone increases, wave-induced lateral surge forces also increase. A floating structure may be modeled as a spring with a natural period of motion in the heave and surge directions. The natural period of motion in a particular direction is inversely proportional to the stiffness of the structure in that direction. As the total mass (including added mass) of the structure increases, the natural periods of motion of the structure become longer.
One method for providing stability is by mooring the structure with vertical tendons under tension, such as in tension leg platforms. Such platforms are advantageous, because they have the added benefit of being substantially heave restrained. However, tension leg platforms are costly structures and, accordingly, are not feasible for use in all situations.
Self-stability (i.e., stability not dependent on the mooring system) may be achieved by creating a large waterplane area. As the structure pitches and rolls, the center of buoyancy of the submerged hull shifts to provide a righting moment. Although the center of gravity may be above the center of buoyancy, the structure can nevertheless remain stable under relatively large angles of heel. However, the heave seakeeping characteristics of a large waterplane area in the wave zone are generally undesirable.
Inherent self-stability is provided when the center of gravity is located below the center of buoyancy. The combined weight of the superstructure, hull, payload, ballast and other elements may be arranged to lower the center of gravity, but such an arrangement may be difficult to achieve. One method to lower the center of gravity is the addition of fixed ballast below the center of buoyancy to counterbalance the weight of the superstructure and payload. Structural fixed ballast such as pig iron, iron ore, and concrete, are placed within or attached to the hull structure. The advantage of such a ballast arrangement is that stability may be achieved without adverse effect on seakeeping performance due to a large waterplane area.
Self-stable structures have the advantage of stability independent of the function of the mooring system. Although the heave seakeeping characteristics of self-stabilizing floating structures are generally inferior to those of tendon-based platforms, self-stabilizing structures may nonetheless be preferable in many situations due to higher costs of tendon-based structures.
Prior art floating structures have been developed with a variety of designs for buoyancy, stability, and seakeeping characteristics.
Various spar buoy designs as examples of inherently stable floating structures in which the center of gravity (“CG”) is disposed below the center of buoyancy (“CB”). Spar buoy hulls are elongated, typically extending more than six hundred feet below the water surface when installed. The longitudinal dimension of the hull must be great enough to provide mass such that the heave natural period is long, thereby reducing wave-induced heave. However, due to the large size of the spar hull, fabrication, transportation and installation costs are increased. It is desirable to provide a structure with integrated superstructure that may be fabricated quayside for reduced costs, yet which still is inherently stable due to a CG located below the CB.
Offshore platform that employs a retractable center column, wherein the center column is raised above the keel level to allow the platform to be pulled through shallow waters en route to a deep water installation site. At the installation site, the center column is lowered to extend below the keel level to improve vessel stability by lowering the CG. The center column also provides pitch damping for the structure. However, the retractable center column adds complexity and cost to the construction of the platform.
Other offshore system hull designs are known in the art. For instance, some offshore system hull designs have an octagonal hull structure with sharp corners and steeply sloped sides to cut and break ice for arctic operations of a vessel. Unlike most conventional offshore structures, which are designed for reduced motions, some structures are designed to induce heave, roll, pitch and surge motions to accomplish ice cutting.
Other designs disclose a drilling and production platform with a cylindrical hull. This structure has a CG located above the CB and therefore relies on a large waterplane area for stability, with a concomitant diminished heave seakeeping characteristic. Although, the structure has a circumferential recess formed about the hull near the keel for pitch and roll damping, the location and profile of such a recess has little effect in dampening heave.
It is believed that none of the offshore structures of prior art are characterized by all of the following advantageous attributes: symmetry of the hull about a vertical axis; the CG located below the CB for inherent stability without the requirement for complex retractable columns or the like, exceptional heave damping characteristics without the requirement for mooring with vertical tendons, and the ability for quayside integration of the superstructure and “right-side-up” transit to the installation site, including the capability for transit through shallow waters. A buoyant offshore structure possessing all of these characteristic is desirable.
Further, there is a need for improvement in offloading systems for transferring petroleum products from an offshore production and/or storage structure to a tanker ship. According to the prior art, as part of an offloading system, a small catenary anchor leg mooring (CALM) buoy is typically anchored near a storage structure. The CALM buoy provides the ability for a tanker to freely weathervane about the buoy during the product transfer process.
For example, an example of a buoy in an offloading system, wherein the buoy is anchored to the seabed so as to provide a minimum weathervane distance from the nearby storage structure. One or more underwater mooring tethers or bridles attach the CALM buoy to the storage structure and carry a product transfer hose therebetween. A tanker connects to the CALM buoy such that a hose is extended from the tanker to the CALM buoy for receiving product from the storage structure via the CALM buoy.
It would be advantageous for an offshore production and/or storage structure to provide the capability to receive a tanker or other vessel and have that vessel moor directly thereto with the ability for the vessel to freely weathervane about the offshore structure while taking on product. Such an arrangement obviates the need for a separate buoy and provides enhanced safety and reduced installation, operating and maintenance costs. The present embodiments meet these needs.
The present embodiments are detailed below with reference to the listed Figures.